1. Field of the Invention
The present invention relates to a variable capacity exhaust gas turbocharger provided with an inner scroll gas passage and an outer scroll gas passage.
2. Background of the Invention
An exhaust gas turbocharger of a relatively small size is used for an internal combustion engine of vehicle use; the exhaust gas turbocharger is provided with a scroll passage is conventionally used; thereby, the exhaust gas emitted by the engine is charged into the scroll passage that is formed in a turbine housing in which a plurality of nozzle vanes is arranged on the inner periphery side of the scroll passage; after passing through the nozzle vanes, the exhaust gas works on the turbine rotor that is provided inside of the circumference formed by the nozzle vanes. Such a turbocharger of a variable capacity type (a variable nozzle area type) as well as of a radial gas-flow type finds widespread acceptance (see the patent references 1 and 3, for instance).
FIG. 8 depicts a general example of conventional exhaust gas turbochargers of a variable capacity type (having an inner scroll passage and an outer scroll passage); thereby, FIG. 8 shows a bird' eye view regarding the variable capacity exhaust gas turbocharger of a conventional type, the turbocharger being depicted by use of an exploded view. As shown in FIG. 8, the turbocharger is provided with: a turbine housing 1; a bearing housing unit 5 coupled to the turbine housing 1; and, a compressor housing 2 coupled to the bearing housing unit 5. Further, in the space that involves the bearing housing unit 5 and the turbine housing 1, a turbine wheel 3 and a turbine stator 4 are housed; incidentally, a compressor wheel is provided in the turbocharger (in the compressor housing) in response to the turbine wheel, although the compressor wheel is not shown. An opening is formed on an upper side of the turbine housing 1; a valve cover 7 is fastened to the turbine housing so as to cover the opening with a gasket 6 between the valve cover 7 and the turbine housing; just below the valve cover, a flow rate control valve 8 is arranged inside the turbine housing 1. The turbine housing 1 is provided with a plate type inlet flange 1a that is formed so as to fix the turbine housing 1 to an exhaust gas outlet side of the engine.
When the turbocharger which exploded view is depicted in FIG. 8 is assembled and the turbocharger-mounted engine is operated under a low speed operation condition, the exhaust gas enters an gas inlet opening inside of the plate type inlet flange 1a (enters the exhaust gas inlet opening of the turbocharger) and the exhaust gas streams along an inner scroll passage T1 that forms a spiral shape passage, as shown in FIG. 9(a); thereby, the streamlines of the exhaust gas flow along the inner scroll passage are directed toward the inner side of the spiral scroll. Further, the turbine housing comprises an outer scroll passage T2 beside the inner scroll passage T1; the inner scroll passage T1 and the outer scroll passage T2 are demarcated by a plurality of blades 4a of a turbine stator; since the stream lines of the exhaust gas flow along the inner scroll passage are directed toward the inner side of the spiral scroll, the exhaust gas flow in the inner scroll passage T1 does not enter the outer scroll passage T2.
On the other hand, when the engine is operated under a high-speed operation condition, a flow rate control valve 8 is pivotally rotated so as to move toward a valve cover 7 as shown in FIG. 9(b); thus, the exhaust gas that enters the turbocharger (the gas inlet opening inside of the plate type inlet flange 1a) streams along the inner scroll passage T1 as well as the outer scroll passage T2. In such a manner as shown in FIG. 9(a), the exhaust gas that enters the inner scroll passage T1 streams along the inner scroll passage T1, the streamlines being directed toward the inner side of the spiral scroll. On the other hand, the exhaust gas that enters the outer scroll passage T2 streams along the spiral of the outer scroll passage T2; and, the exhaust gas streams into the inner scroll passage through the slit area (or the throat area) between a blade 4a of the turbine stator and an adjacent blade; thus, the exhaust gas flow in the outer scroll passage joins the exhaust gas flow in the inner scroll passage. Thus, a merging gas flow is formed and streams toward the turbine wheel; then, after passing by the turbine wheel, the exhaust gas is discharged outside of the turbine housing 1.
Incidentally, when the engine is operated under the low speed operation condition as depicted in FIG. 9(a), the exhaust gas streams only in the inner scroll passage T1; thus, the exhaust gas flow speed under the low speed operation condition is faster than that under the high speed operation condition. On the other hand, when the engine is operated under the high speed operation condition as depicted in FIG. 9(b), the exhaust gas streams in and along the inner scroll passage T1 as well as the outer scroll passage T2; thus, the back pressure regarding the exhaust gas flow is reduced and the fuel consumption is improved.
However, in the variable capacity exhaust gas turbocharger of conventional technologies, there exist the components such as the turbine stator 4, the flow rate control valve 8 and the valve cover; accordingly, the heat capacity (or heat accumulation) regarding the components that form the turbine housing assembly becomes large; thus, the components forming the turbine housing assembly absorb large heat quantity during an engine start transition condition, namely, during a time interval from the time point of the engine start in a cold condition to the time point of the temperature distribution stabilization in a hot operating condition. Hence, it requires longer time for the temperature of the catalyst in the exhaust gas purifying device provided on a downstream side of the exhaust gas outlet of the turbine housing to reach the activation temperature regarding the catalyst. In this way, it is required that the heat capacity of the components that form the turbine housing assembly be reduced. In order to enhance the purifying efficiency, it is preferable to reduce the heat capacity of the turbine housing assembly as small as possible.
Further, the configuration in which the valve cover 7 is included is not preferable in view of costs, time and labor in packaging, and possible gas leakage and so on.